One-time programmable memories with low power read operation and novel sensing scheme

ABSTRACT

A time-based sensing circuit to convert resistance of a one-time programmable (OTP) element into logic states is disclosed. A one-time programmable (OTP) memory has a plurality of OTP devices. At least one of the OTP devices can have at least one OTP element that is selectively accessible via a wordline and a bitline. The bitline can be coupled a capacitor and the capacitor can be precharged and discharged. By comparing the discharge rate of the capacitor to discharge rate of a reference capacitor in a reference unit (e.g., reference cell, reference resistance, reference selector, etc.), the PRE resistance can be determined larger or smaller than a reference resistance and then converting the OTP element resistance into a logic state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. patent application Ser. No. 16/559,560, filed on Sep. 3, 2019 and entitled “PROGRAMMABLE RESISTIVE MEMORIES WITH LOW POWER READ OPERATION AND NOVEL SENSING SCHEME,” which is hereby incorporated herein by reference, which in turn is a Continuation-In-Part of U.S. patent application Ser. No. 15/953,422, filed on Apr. 14, 2018 and entitled “LOW POWER READ OPERATION FOR PROGRAMMABLE RESISTIVE MEMORIES,” which is hereby incorporated herein by reference, which claims priority benefit of U.S. Provisional Patent Application No. 62/485,895, filed on Apr. 14, 2017 and entitled “CIRCUIT AND SYSTEM OF ULTRA LOW VOLTAGE AND LOW CURRENT READ FOR PROGRAMMABLE RESISTIVE MEMORIES,” which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a sensing circuit for converting resistance in programmable resistive devices into logic states. The programmable resistive device memories are OTP, MTP, PCRAM, FeRAM, RRAM, and MRAM, etc.

Description of the Related Art

A Programmable Resistive Device (PRD) is generally referred to a device whose resistance can be changed by means of programming. Resistance states can also be determined by resistance values. For example, a programmable resistive device can be a One-Time Programmable (OTP) device, such as electrical fuse (or anti-fuse), and the programming means can apply a high voltage to induce a high current to flow through the OTP element. When a high current flows through an OTP element (e.g., by turning on a selector), the OTP element can be programmed, or burned into a high or low resistance state (depending on either fuse or anti-fuse).

FIG. 1 shows a schematic diagram of a conventional Programmable Resistive Device (PRD) 10 that has a Programmable Resistive Element (PRE) 11 and a selector 12. The PRE 11 can be an electrical fuse, anti-fuse, floating-gate device, phase-change material, resistive RAM element, or Magnetic Tunnel Junction (MTJ), etc. The selector 12 can be MOS, diode, MOS/diode merged, or even bipolar device that can be turned on or turned off by asserting a signal Sel.

The resistance of a PRE in a PRD device needs to be converted into a logic level after reading the PRD cell. This can be achieved by using a sensing circuit or sense amplifier (SA). The conventional way of sensing a PRE resistance is to convert the resistance value into a voltage by applying a current flowing through the PRE, then using a voltage amplifier to enlarge the voltage signal. This scheme depends on amplification of a MOS device that usually needs to be biased fully using a high voltage that consumes substantial amount of current to have high gain.

FIG. 2 is a schematic diagram of a conventional sense amplifier 20. The SA 20 has a PMOS 21 with the gate coupled to the drain. The gate of PMOS 21 is also coupled to a gate of PMOS 22. The sources of PMOS 21 and 22 are coupled to a supply voltage VDD. The drains of PMOS 21 and 22 are coupled to drains of NMOS 23 and 24, respectively. The gates of NMOS 23 and 24 are coupled to a differential input signal, Vp and Vn, respectively. The sources of NMOS 23 and 24 are coupled to a drain of a NMOS 26, whose gate is coupled to an enable signal ϕ and whose source is coupled to ground. The input differential voltage between Vp and Vn can be amplified to the output Vout, at the drain of PMOS 22.

FIG. 3 is a schematic diagram of another conventional voltage sense amplifier 30. The scheme is to bias MOS devices in a latch into high gain region and then latching. Biasing MOS in high gain region normally requires substantial amount of current and operating in high voltage. A PMOS 31 has a source coupled to a supply voltage VDD, a drain coupled to a drain of an NMOS 33, whose source is coupled to a drain of another NMOS 35. The gate of NMOS 33 is coupled to an input voltage V+. The gate of NMOS 35 is coupled to the gate of PMOS 31. There is also another similar branch that has a PMOS 32 having a source coupled to a supply voltage VDD, a gate coupled to the drain of PMOS 31, a drain coupled to a drain of a NMOS 34, whose source is coupled to a drain of another NMOS 36. The gate of NMOS 34 is coupled to another input voltage V−. The gate of NMOS 36 is coupled to the gate of PMOS 32. The four devices 31, 35, 32, and 36 are interconnected like two cross-coupled inverters, i.e. the gates of 31 and 35 are coupled to the drain of PMOS 32, and the gates of 32 and 36 are coupled to the drain of PMOS 31. The sources of the NMOS 35 and 36 are coupled to a drain of an NMOS 39, whose gate is coupled to an enable signal ϕ and whose source is coupled to ground. The input differential signals of V+ and V− can be amplified to Vn and Vp, respectively, by turning on the enable signal ϕ. In this embodiment, the input devices 33 and 34 are placed in the middle of the three MOS device stack. The input device 33 and 34 can be placed in top or bottom of the MOS stack in the other embodiments. There are also other embodiments of resistance sensing by replacing NMOS 33 and 34 by reference and cell resistance directly, respectively.

The conventional approaches to voltage sensing in FIG. 2 or FIG. 3 depend on the supply voltage VDD and MOS threshold voltages Vtn and Vtp to bias the MOS devices in high gain region, which normally needs some amount of current. If the supply voltage is 0.8V and the Vtn and |Vtp| are about 0.3V, sensing the PRD with only 0.4V would be very difficult for MOS to be in high gain regions. Even if the voltage sense amplifier was able to operate, the MOS bias current in high gain region would be substantially high for a sense amplifier (SA) to work. At least 100 uA would be needed to do so and achieving 1 uA of sensing current would be almost impossible. In IoT applications, there are some requests for sensing programmable resistive devices with only 0.4V voltage and 1 uA current, while the nominal supply voltage is 0.8V.

The conventional approaches to sensing a resistance by converting into voltage require high supply voltage and high current. However, the industry trend in semiconductors is use of low supply voltage and low current consumption, especially in PC, tablet, smart phone, portable, and IoT applications. Thus, there is a continuing need for improved approaches to sense resistance of programmable resistive memory cells under low voltage and low current conditions.

SUMMARY OF THE INVENTION

Embodiments of sensing programmable resistive device cells in low supply voltage with low current are disclosed. The methods and circuits of low voltage and low current sensing can be used in any kind of programmable resistive memories, such as OTP, MTP, PCRAM, RRAM, FeRAM, and MRAM, etc. One or more of the embodiments disclosed herein can include or utilize techniques and circuitry that converts sensed resistance of programmable resistive devices into logic states.

In one embodiment, a capacitor can be charged to near a supply voltage level during a first period of time. Then, the capacitor can be discharged through a resistance over a second period of time. A voltage at the discharging capacitor can be compared with a reference voltage to change a memory read output. The time for an output logic device (e.g., comparison device) to change its output is determined by a product of the resistance and capacitance. Since the capacitance is or can be known, the time for the output logic device to change the output is related to the resistance value. Thus, the resistance can be determined by the time delay since start of the discharging. The longer the time delay, the larger the resistance. In one implementation, MOS devices are only used as switches to turn on or off for charging or discharging. Therefore, the threshold voltages of the MOS devices are not crucial in this time-based sensing scheme.

The invention can be implemented in numerous ways, including as a method, system, device, or apparatus (including computer readable medium). Several embodiments of the invention are discussed below.

As a programmable resistive memory, one embodiment can, for example, include at least a plurality of programmable resistive device (PRD) cells. At least one of the PRD cells can include at least: a selector controlled by a control signal, the selector having a at least a first end and a second end, the second end being coupled to a first conductive line; a programmable resistive element (PRE) having a first end coupled to a capacitor and a second end coupled to the first end of the selector; and a logic device can be coupled between the first end of the PRE and the capacitor as a switch or multiplexer. The charge/discharge rate of the capacitor can be used to determine the logic state of the PRE.

As an electronic system, one embodiment can, for example, include at least a processor, and a programmable resistive memory operatively connected to the processor. The programmable resistive memory can include at least a plurality of programmable resistive device (PRD) cells for providing data storage, each of the PRD cells can include at least: a selector controlled by a control signal, the selector having a first end and a second end, the second end being coupled to a first conductive line; a programmable resistive element (PRE) having a first end coupled to a capacitor and a second end coupled to the first end of the selector; and a logic device coupled between the first end of the PRE and the capacitor as a switch or multiplexer. The charge/discharge rate of the capacitor can be used to determine the logic state of the PRE.

As a method for providing a programmable resistive memory including a plurality of programmable resistive device cells and at least one capacitor. At least one of the programmable resistive device cells includes at least a programmable resistive element. One embodiment can, for example, include at least: charging the capacitor; ceasing the charging; subsequently coupling the programmable resistive element to the capacitor; monitoring a discharge rate of the capacitor while the programmable resistance element remains coupled to the capacitor; determining a resistance value of the programmable resistive element based on the monitoring of the discharge rate; and determining a logic state for the at least one of the programmable resistive device cells based in the determined resistance value of the programmable resistive element.

As a One-Time Programmable (OTP) memory, one embodiment can, for example, include at least a plurality of OTP cells, each of the OTP cells including an OTP element coupled to a selector, the selector having a control signal; a plurality of wordlines, each coupled to a plurality of the OTP cells via the control signal of the selector; a plurality of bitlines, each coupled to a plurality of at least one end of the OTP cells; at least one reference OTP cells, each of the reference OTP cells including a reference OTP element coupled to a reference selector, the reference selector having a control signal; at least one reference wordline, each coupled to the reference OTP cells via the control signal of the reference selector; and at least one reference bitline, each coupled to at least one end of the reference OTP cell. The bitlines and reference bitlines can be pre-charged and then are discharged at substantially the same time. The resistance of the OTP element is able to be determined by comparing the discharge rates of the bitline versus the reference bitline.

As an electronic system, one embodiment can, for example, include at least a processor; and a one-time programmable (OTP) memory operatively connected to the processor. The OTP memory includes at least a plurality of OTP cells for providing data storage. Each of the OTP cells can include at least: a one-time programmable (OTP) element coupled to a selector that has an enable signal; a plurality of wordlines coupled to the enable signals of the selector; a plurality of bitlines coupled to a first end of OTP cells; at least one reference OTP cell comprising a reference OTP resistor coupled to a reference selector that has a reference enable signal; at least one reference wordline coupled to the enable signal; and at least one bitline coupled to at least a first end of the reference OTP cells. Both the bitline and reference bitline are able to be pre-charged to one voltage supply line and discharged to another supply voltage at substantially the same time. The resistance of the OTP can be determined by comparing the discharge rates of the bitline and the reference bitline.

As a method for operating a one-time programmable (OTP) memory, one embodiment can, for example, operate the OTP memory to provide data storage. The OTP memory can, for example, include: a plurality of OTP cells, at least one of the OTP cells including a plurality of a OTP element and a selector, the OTP element coupled to a selector, a plurality of wordlines coupled to the enable signal of the selector of at least one OTP cell, a plurality of bitlines coupled to at least a first end of the OTP cells, at least one reference OTP cell, the at least one of the reference OTP cell including at least one reference OTP element coupled to a reference selector, at least one reference wordline coupled to the enable signal of the reference selector of at least one reference OTP cell, and at least one reference bitline coupled to at least a first end of the reference resistance cell. The method can, for example, include at least: charging the bitline and the reference bitline; ceasing charging the bitline and the reference bitline substantially at the same time; subsequently coupling the OTP element and the reference OTP element to the bitline and reference bitline, respectively; monitoring a discharge rate of the bitline and the reference bitline to reach a predetermined voltage threshold; and determining a logic state for the at least one of the OTP cells based on the discharge rates of the bitline to the reference bitline.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the following detailed descriptions in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:

FIG. 1 shows a schematic diagram of a conventional programmable resistive memory device.

FIG. 2 shows a schematic diagram of a conventional voltage sense amplifier.

FIG. 3 shows a schematic diagram of another conventional voltage sense amplifier.

FIG. 4(a) illustrates a schematic diagram of a low power sensing circuit according to one embodiment.

FIG. 4(b) illustrates a timing waveform of a voltage being discharged and the comparator output according to one embodiment.

FIG. 5(a) illustrates a schematic diagram of time-based sensing circuit according to one embodiment.

FIG. 5(b) illustrated a timing waveform of discharging capacitor voltages in a cell unit and a reference unit, according to one embodiment.

FIG. 5(c1) illustrates a schematic diagram of a portion of a PRE memory utilizing time-base sensing, using at least one reference column according to one embodiment.

FIG. 5(c2) illustrates a schematic diagram of a portion of a PRE memory utilizing time-base sensing, using at least one reference column according to another embodiment.

FIG. 5(d) illustrates a schematic diagram of a portion of a PRE memory utilizing time-base sensing using reference rows, according to another embodiment.

FIG. 5(e) illustrates a schematic diagram of a portion of a PRE memory utilizing self-timed circuit for pre-charge and discharge.

FIG. 6 is a flow diagram of a method of converting resistance into time according to one embodiment.

FIG. 7 is a flow diagram of a method of time-based resistance sensing according to one embodiment.

FIG. 8 shows a processor system with at least one programmable resistive memory using low voltage and low current sensing circuit, according to one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments disclosed herein use capacitors discharging through programmable resistance element (PRE) to evaluate a programming state. By comparing a discharging time to reach a predetermined voltage, a resistance of the PRE can be converted into a logic state. The discharging time can be compared with a discharge time provided by a separate unit that used a reference resistance.

The programmable resistive element and a sensing circuit can be included in a Programmable Resistive Device (PRD) memory. Optionally, the programmable resistive element and the sensing circuit can be included within an electronic system.

The PRD can be a One-Time Programmable (OTP) device, such as electrical fuse (or anti-fuse). A current that flows through an OTP element (e.g., by turning on a selector) can program the OTP element or can burn the OTP element into a high or low resistance state (depending on either fuse or anti-fuse). The electrical fuse can be an interconnect or contact/via fuse. The interconnect fuse can be made of MOS gate, polysilicon, silicide, silicided polysilicon, metal, metal alloy, local interconnect, MOS gate, or thermally isolated active region. The contact or via fuse can be made of a single or a plurality of contact or via holes for programming. The anti-fuse can be made of breaking down a MOS oxide or a dielectric between conductors. The PRD can also be other kinds of memory devices, such as MTP, PCRAM, RRAM, FeRAM, and MRAM, etc. that can be programmed more than once.

Any MOS devices in this sensing circuit are used as switches. Therefore, their threshold voltages related to the supply voltage are not crucial to the operation of the circuit. Thus, low voltage and low current sensing can be achieved in standard CMOS logic process without additional masks or process steps to save costs.

FIG. 4(a) illustrates a schematic diagram of a portion of a low power sensing circuit 50 according to one embodiment. The sensing circuit 50 has a switch 52 coupled to a supply voltage VDD and to ground through a capacitor 51. The capacitor 51 is coupled to a switch 54 then to a resistive device 53 to ground. The capacitor 51 can be charged to VDD by turning on the switch 52 and turning off the switch 54. After the capacitor 51 is fully charged, the capacitor 51 can be discharged by turning on the switch 54 and turning off the switch 52. Thus, the voltage at node A at the capacitor 51 can be discharged to ground according to a RC-time constant. The RC-time constant is dependent on the product of resistance (R) and capacitance (C). Since the capacitance (C) of the capacitor 51 can be fixed, the discharge rate will depend on resistance (R) of the resistive device 53.

FIG. 4(b) illustrates a timing waveform of a voltage at node B being discharged according to one embodiment. If the resistance of the resistive device 53 has a larger resistance than a reference resistance, the discharge curve would look like 62. Conversely, if the resistance of the resistive device 53 has a smaller resistance than the reference resistance, the discharge curve would look like 61. If one-half VDD (Vdd/2) is the predetermined voltage, time delays ts and tl for curves 61 and 62, respectively, to reach the predetermined voltage are as shown in FIG. 4(b) as logic waveforms 63, and 64 respectively. The larger the resistance, the longer the time delay. In other words, the sensing circuit 50 converts resistance into time, as is depicted in FIG. 4(b). If a comparator (not shown) is coupled to node B, the discharge waveforms of 61 and 62 after comparaison can be raised to VDD as logic waveforms 63 and 64, respectively, after delay times ts and tl, respectively. By comparing the delay times ts and tl with a reference delay time, the resistance of the resistive device 53 can be determined.

FIG. 5(a) illustrates a portion of a schematic diagram of time-based sensing circuit 100, according to one embodiment. The time-based sensing circuit 100 has a cell unit 120 and a reference unit 130. The cell unit 120 has a capacitor 101 coupled to a supply voltage VDD through a charging pass gate 102 and coupled to a ground. The capacitor 101 is also coupled to a discharge pass gate 106. The discharge pass gate 106 couples to a programmable resistive device 103. The programmable resistive device 103 includes a programmable resistive element 104 coupled in series with a selector 105 that couples to ground. The discharge capacitor 101 is also coupled to an inverter 107, which can serve as a comparator or logic state converter.

The reference unit 130 has a capacitor 111 coupled to a supply voltage VDD through a charging pass gate 112 and coupled to a ground. The capacitor 111 is also coupled to a discharge pass gate 116. The discharge pass gate 116 couples to a reference device 113. The reference device 113 includes a reference element 114 coupled in series with a selector 115 that couples to ground. The capacitor 111 is also coupled to an inverter 117, which can serve as a comparator or a logic state converter. The output of the inverters 107 and 117 can be coupled to an input and an enable a latch (or flip-flop) 108, respectively. The output QB of the latch (or flip-flop) 108 can represent a logic state of the PRE 104 with respect to the reference element 114.

The operation of time-based sensing circuit can be further elaborated as follows. The cell unit 120 includes the capacitor 101 and the reference unit 130 includes the capacitor 111. The capacitors 101 and 111 typically have the same capacitance C. The capacitors 101 and 111 can be charged to near VDD with assertion of CKB which turns on the charge pass gates 102 and 112, while the discharge pass gates 106 and 116 are turned off. When the capacitors 101 and 111 are nearly full charged, the charge pass gates 102 and 112 can be turned off and the discharge pass gates 106 and 116 as well as the selectors 105 and 115 can be turned on. The capacitors 101 and 111 will then be respectively discharged through the resistance of the PRE 104 and the reference element 114. The capacitors 101 and 111 are also coupled to inverters 107 and 117, respectively, that serve as logic comparators to generate logic states. Before discharge, the outputs of the inverters 107 and 117, as comparators, are all Os. When the capacitor voltages have discharged below the trip points of the inverters 107 and 117, the outputs of the inverters 107 and 117 will be independently changed to 1 s. If the output of the inverter 117 is changed to 1, the latch (or flip-flop) 108 will be turned on to latch the output of the inverter 107. The output QB will be high if the resistance of the PRE 104 in the cell unit 120 has lower resistance than the resistance of reference element 114; otherwise, the latch (or flip-flop) 108 output QB will be low.

FIG. 5(b) illustrates a timing waveform 200 of the capacitor voltages at nodes Bc and Br in the cell unit 120 and the reference unit 130, respectively, according to one embodiment. A curve 202 shows discharging of the capacitor 111 in the reference unit 130. When the voltage at node Br reaches half VDD (Vdd/2), which is set as the trip point of the inverter 117, the output of the inverter 117 will become high. Then, the latch (or flip-flop) 108 can latch data 1 or 0 for curves 201 or 203, respectively, depending on whether the resistance of the PRE 104 in the cell unit 120 is lower or higher than the resistance of the reference element 114. Waveforms 211, 212, and 213 are comparator outputs of 201, 202, and 203, respectively.

FIG. 5(c1) illustrates a schematic diagram of a portion of a block diagram 500 of a PRE macro incorporating the time-based sensing circuit, according to one embodiment of the present invention. The schematic diagram 500 has a plurality of PRE cells 510 organized as n row by m columns arrays and one additional reference column. Each PRE cell 510 has a PRE element 511 coupled to a selector 512. One end of PRE 510 is connected as a bitline (BL) in each column and is coupled to a Y-write Pass Gate (YWPG) 540 that further couples to VDDP for programming. Each bitline is also coupled to one or more of Y-read pass gate (YRPG) 530 as multiplexers to dataline (DL) and then each DL is coupled to a discharged capacitor 535 and a comparator 540. In another embodiment, each DL can be coupled to a plurality of bitlines.

Additionally, there is an additional reference resistor 551 coupled to the reference bitline (BLR) in a reference column. The comparator output of the normal column can be coupled to an input D of a latch 550, while the comparator output of the reference column can be coupled to the enable E of the latch 550. There can be PMOS pull-ups (not shown) to pre-charge DL0-DLm−1 and DLR to VDD before sensing. During sensing, the DL pre-charges are turned off, YRPG and WL are turned on, the selected DL in DLi (i=0, 1, . . . , m−1) and DLR will be discharged to ground through PRE element 511 and the reference resistance 551, respectively. When the DL and DLR voltage fall below a pre-determined threshold, the comparator output Ci (i=0, 1, . . . m−1) and CR will change logic state. CR can be used as a latch enable to latch the one or more Ci (i=0, 1, . . . m−1) to determine if the resistance of the PRE element is larger or smaller than the reference resistance.

FIG. 5(c2) illustrates a schematic diagram of a portion of a block diagram 500″ of a PRE macro incorporating the time-based sensing circuit, according to another embodiment. The block diagram 500″ has a plurality of PRE cells 510″ organized as n row by m columns arrays and one additional reference column BLR. Each PRE cell 510″ has a PRE element 511″ coupled to a selector 512″. The other end of PRE 510″ connected as a bitline (BL) in each column is coupled to a Y-write Pass Gate (YWPG) 540″ that further couples to VDDP for programming. Each bitline is also coupled to one or more of Y-read pass gate (YRPG) 530″ as multiplexers to a single dataline (DL0). In this embodiment, all bitlines are multiplexed to a single dataline (DL0). Also, discharge capacitor 535″ can be omitted, if the DL0 capacitance is larger enough. There is an additional reference resistor 551″ coupled to the reference bitline in the reference column BLR. The comparator output of the normal column is coupled to an input D of a latch 550″, while the comparator output of the reference column is coupled to the enable E of a latch 550″. There can be PMOS pull-ups (not shown) to pre-charge DL0 and DLR to VDD before sensing. During sensing, the DL0/DLR pre-charge is turned off, YRPG 530″, reference YRPG 531″ and WL are turned on, and the selected BLi (i=0, 1, . . . , m−1) and BLR will be discharged to ground through PRE element 511″ and the reference resistance 551″, respectively. When the DL0 and DLR voltage fall below a pre-determined threshold, the comparator outputs C0 and CR will change logic state. CR can be used as a latch enable to latch C0 to determine if the resistance of the PRE element is larger or smaller than the reference resistance.

The time-based sensing circuits 500′ and 500″ shown in FIGS. 5(c1) and 5(c2), respectively, are self-timed circuits. The reference column is placed in far right, for example, so that the reference cells in each wordline (WL) will be turned on the last. Dummy YRPG 530″ or reference YRPG 531″ can be placed to match the impedance in the normal bitlines, or as a multiplexer, respectively. During discharging, the reference comparator 541″ can drive the enable signals of all latches to latch the cell data. Further, the signal CR can also be used to shut off the WL or YRPG to save power consumption. In these embodiments, the time delays of the cell and reference units can be tracked accurately. The reference cells can be placed in rows, rather than in columns as shown in FIG. 5(c1) or 5(c2).

FIG. 5(d) illustrates a schematic diagram of a portion of a block diagram 500′ of a PRE macro incorporating the time-based sensing circuit, according to one embodiment. The block diagram 500′ has a plurality of PRE cells 510′ organized as n row by m columns arrays and two additional reference rows WLRe and WLRo. Each PRE cell 510′ has a PRE element 511′ coupled to a selector 512′. One end of PRE 510′ can be connected as bitline (BL) in each column and can be coupled to a Y-write Pass Gate (YWPG) 540′ that further coupled to VDDP for programming. Each bitline is also coupled to one or more of Y-read pass gate (YRPG) 530′ as multiplexers to dataline (DL) and then each DL is coupled to a discharged capacitor 535′ and a comparator 540′. Each wordline (WL) is only connected to half of the cells. For example, the even WLs only connected to even columns cells and odd WLs only connected to odd columns cells. The two additional reference wordlines WLRe and WLRo have reference resistance in the cell are also connected in the same way as normal WLs. If even WL is turned on, the odd reference WL is also turned on and vice versa. By doing it this way, if one column has a cell turned on, the other column has a reference cell turned on, e.g. WLe and WLRo, which will be turned on at the same time. This is called self-reference in one pair of columns, i.e. one is coupled to a normal cell in the even column and the other coupled to a reference cell in an odd column. Therefore, the comparator outputs can be used to comparing the timing delays so as to determine cell resistance depending on which unit will reach VDD earlier. This can be done by coupling the comparator output of the cell unit to a latch input D and the reference unit to the latch enable E. For example, if the even cell in C0 is accessed, C0 is coupled to the latch input D and C1 is coupled to the enable input E of the latch 550′ and vice versa. Another latch 552′ does just the opposite. The two latches 550′ and 552′ are further multiplexed in an output multiplex 580′ to select the proper output to generate Q0 and Q1. There are PMOS pull-ups (not shown) to precharge DL0-DLm−1 and DLR to VDD before sensing. During sensing, the DL pre-charges are turned off, YRPG and WL are turned on. At least one pair of DL will be discharged to ground through PRE element 511′ and the reference resistance 551′, respectively. When the DL voltages fall below a pre-determined threshold, the comparator output of a pair of column will change logic states. The comparator outputs C0 and C1 are coupled to input and enable of a latch to determine the PRE resistance. In this embodiment, the cell and reference branches can be tracked even better than in the reference column embodiment shown in FIG. 5(c1). The reference rows can be more than one pair to provide different reference resistance levels to be chosen from in the other embodiments. In another embodiment, the number of latches in a pair of column can be one with proper logic to determine proper cell and reference column so as to interpret the latch input and enable, respectively.

FIG. 5(e) shows a portion of a circuit in schematics 500′″ to further illustrate a self-timed circuit. The circuit 500′″ has a plurality of bitlines (BL), 570_0′″ through 570_m−1′″, or BL0 through BLm−1. Each bitline, BL0 through BLm−1, is coupled to at least one Y-Read Pass Gate (YRPG), 530_0′″ through 530_m−1′″, as a multiplexer. The outputs of the YRPGs are coupled to at least one dataline (DL). In this circuit, k datalines, DL0 through DLk−1, are outputs of the multiplexers YRPGs, 530_0′″ through 530_m−1′″, from m-bitline, BL0 through BLm−1, inputs for sensing. Each dataline is coupled to at least one pre-charge PMOS 520′″, capacitor 535′″ and comparator 540_0′″ through 540_m−1. The same with the reference branch that includes at least one reference bitline BLR 571″, reference YRPG 531′″, reference dataline DLR, reference capacitor 536′″, and reference comparator 541′″, which are coupled to each other in similar configurations. For example, the DLR is coupled to the reference capacitor 536′″ and the reference comparator 541′″, which has an output CR. The CR is coupled to an input of NOR 590″; with the output coupled to the gates of PMOS pull-ups 520_0′″ through 520_k−1″; and reference PMOS pull-up 521′″. The other input of the NOR 590′″ is coupled to CKB to turn on/off all PMOS pull-ups. The output CR of the reference comparator 541′″ can also coupled to an input of a plurality of NAND gates 580_0′″ through 580_n−1′″ with the outputs coupled to a wordlines (WL), WL0 through WLn−1, each. The other input of the NAND gates are coupled to X-pre-decoders or decoders, such as XPDi or XPDj, where 0<=l, . . . , j . . . <=n−1, as shown. The cell array and the output latches are not shown for simplicity.

During pre-charging, all XPDs and YRPGs 530_0″ through 530_m−1″; and 531′″ are low and turned off, respectively. DL0 through DLk−1 and DLR can be pre-charged high to VDD by setting CKB high to turn on all PMOS pull-ups, 520_0′″ through 520_k−1′″ and 521′″ accordingly. During sensing, the selected X-pre-decoders XPDi (j=0, 1, 2, . . . , n−1) are asserted to turn on a selected wordline, WL 580_0′″ through 580_n−1′″. And at least one of the YRPGs, 530_0′″ through 530_m−1′″ and reference YRPG 531′″ are also turned on. Then, CKB is set to low to turn off all PMOS pull-ups, 520_0″; through 520_k−1″; and 521′″. As a result, the datalines, DL0 through DLk−1, and reference dataline DLR can be discharged through the cell PRE and the reference PRE, respectively. Depending on the resistance of cell PREs and the reference PRE, datalines DL0 . . . DLk−1 and DLR can be discharged at different rates accordingly. When the DLR voltage drops to a pre-determined level, or half VDD for example, the reference comparator output CR will be switched from low to high. CR can be used to latch outputs C0 . . . Ck−1 into latches, where C0, . . . Ck−1 are the outputs of the comparators 540_0′″ through 540_k−1 with DL0 . . . DLk−1 as inputs, respectively. Latches can also be provided such as shown in FIGS. 5(c1), 5(c2), and 5(d), but not shown herein for simplicity. At the same time, CR de-asserts WL drivers 580_0′″ through 580_n−1′″ to prevent further discharging. CR can also turn on the NOR 590′″ for pre-charging and preparing for the next sensing cycle. Thus, pre-charging for the next sensing does not need to start from VSS so that energy can be saved. In other embodiments, the YRPGs can also be turned off after CR is asserted.

The circuits in FIGS. 5(c1), 5(c2), 5(d) and 5(e) are for illustrative purposes. There can be many different but yet equivalent embodiments that still fall within the scope of this invention. The numbers of row and column can vary. The numbers of datalines and sensing units can vary. As examples, there can be 1:1 ratio of bitlines to datalines, or there can be 1 to many ratio of datalines to bitlines. The number of the reference row/column can also vary. The reference row/column can be one per row, one per column, one per I/O, one per section, or can be a group of any rows/columns in a macro. The cells coupled to a wordline can be fully populated or half populated in different embodiments. The selector in the cell can be NMOS, PMOS, diode, merged MOS/diode, or bipolar, etc. The PRE cells, which can be 1R1T cells as noted above, can have a BL coupled to the source of the NMOS selector and the BL can couple to a shared NMOS in a column as YWPG and then further coupled to VSS, while the other end of the PRE can be coupled to VDDP directly in another embodiment. There can be more than one level or none of YWPG between BLs to VDDP for programming. There can be more than one level or none of YRPG between BLs to DLs for reading. In another embodiment, the additional DL capacitors can be omitted, but using intrinsic BL or DL loading as charging/discharging capacitor instead. In another embodiment, comparing time delays between cell and reference units can be during charging phase, instead of discharging phase.

The time-based sensing circuits in FIG. 5(a)-5(e) are merely exemplary embodiments. Many alterations or variations are possible. The pass gates can be embodied as NMOS, PMOS, or full CMOS pass gates. The capacitors can be made of MOS gate capacitor, Poly-Insulator-Poy (PIP), Metal-insulator-Metal (MIM) capacitor, or other type of capacitors. If MOS gate capacitor is used, thick oxide I/O devices are preferred over the core logic devices to reduce leakage current. The comparison devices (e.g., inverters or any logic gate) can be embodied as any kind of analog comparators or sense amplifiers, either static or dynamic comparators. The comparison devices can also be an arrangement of logic gates, such as inverters or Schmitt triggers to switch output when the input reaches certain voltage level. The reference voltage used for comparison can be any voltage between VDD and ground, though half VDD (Vdd/2) is a convenient voltage. The pass gates, e.g., pass gates 106 and 116 in FIG. 5(a), can be omitted in some embodiments. The time to charge capacitors can be made longer than discharging time to reduce peak current in some embodiments, i.e. charging or discharging duty cycle need not be 50%. A transparent latch (or flip-flop) can be used to latch the output data in some embodiments. There can be many different varieties and yet equivalent embodiments and they all fall into the scope of this invention.

FIGS. 6 and 7 show flow charts depicting embodiments of a time-based measuring resistance method 300 and a time-based reading method 400, respectively, for a programmable resistive memory in accordance with certain embodiments. The methods 300 and 400 are described in the context of a programmable resistive memory, such as the programmable resistive memory 500, 500″ and 500′ in FIGS. 5(c1), 5(c2), 5(d), and 5(e). In addition, although described as a flow of steps, one of the ordinary skilled in the art will recognize that at least some of the steps may be performed in a different order, including simultaneously, or skipped.

FIG. 6 is a flow diagram of a method 300 of converting resistance into time according to one embodiment. In the first step 310, a capacitor having a capacitance C is charged to near a supply voltage such as VDD. In the second step 320, the capacitor is discharged through a resistive device having a resistance R. In the third step 325, a fixed voltage is acquired as a reference voltage level Vr. The fixed voltage can be previously determined or determined when needed. In the fourth step 330, compare the discharging voltage with the reference voltage level to determine if the discharging voltage has reached the reference voltage level. In the fifth step 340, the time from starting discharging the capacitance C of the capacitor until discharging voltage is determined to be at or less than the reference voltage level is determined by R*C. Since the capacitance C is fixed, the resistance R can be determined accordingly.

FIG. 7 is a flow diagram of a method 400 of time-based resistance sensing according to one embodiment. The method 400 can use a PRD cell and a reference unit. In the step 410, at least two capacitors are charged to near a supply voltage VDD. One capacitor is part of the PRD cell unit, and another capacitor is part of the reference unit. In the step 420, the two capacitors are discharged through a Programmable Resistive Element (PRE) and a reference element, respectively. The PRD cell includes the PRE, and the reference unit includes the reference element. In one implementation, the discharging can be initiated by turning on selectors. In the step 425, a fixed reference voltage is determined to check the discharging voltage level. In the step 430, the fixed reference voltage can be compared with the discharged voltages by comparators. The comparator outputs can be independently changed when the voltages are discharged below the reference voltage level. In the step 440, the comparator output of the reference unit can be used to turn on a latch (or flip-flop) when the discharge voltage level in the reference unit is below the reference voltage level. In the step 450, the comparator output of the PRD cell is used as an input to the latch (or flip-flop). In the step 460, the output of the latch (or flip-flop) is a logic state for the PRE resistance. In the step 470, the capacitor discharging is disabled. In the step 480, the capacitor and reference capacitor are pre-charged again to prepare for the next sensing.

FIG. 8 shows a processor system 700 according to one embodiment. The processor system 700 can include a programmable resistive device 744 and a time-base sensing circuit 742, in a programmable resistive memory 740, according to one embodiment. The processor system 700 can, for example, pertain to a computer system. The computer system can include a Central Process Unit (CPU) 710, which communicate through a common bus 715 to various memory and peripheral devices such as I/O 720, hard disk drive 730, CDROM 750, programmable resistive memory 740, and other memory 760. Other memory 760 is a conventional memory such as SRAM, DRAM, or flash, typically interfaces to CPU 710 through a memory controller. CPU 710 generally is a microprocessor, a digital signal processor, or other programmable digital logic devices. Programmable resistive memory 740 is preferably constructed as an integrated circuit, which includes a plurality of programmable resistive devices 744 in a memory array and at least one time-base sensing circuit 742. The programmable resistive memory 740 typically interfaces to CPU 710 with simple interface control or through a memory controller. If desired, the programmable resistive memory 740 may be combined with the processor, for example CPU 710, in a single integrated circuit.

If the PRE is an One-Time Programmable (OTP) element, such as an electrical fuse, a method of programming a fuse reliably can include the following steps: (a) starting with a low program voltage initially to program a portion of an OTP memory and incrementing the program voltage until all planned OTP cells can be programmed and verified pass, marked this voltage as a low bound of the program voltage, (b) continuously incrementing the program voltage to program another portion of OTP memory cells until at least one OTP cell, whether programmed or not, is verified failure, marked this voltage as a high bound of the program voltage. Incremental programming can happen on the same or another unprogrammed OTP memories in different embodiments. Furthermore, the program time can be adjusted to characterize the program window by repeating the above steps (a) and (b) accordingly until a low bound, high bound, or program window (voltage range between high and low bound) meets a target value. The window of programming an electrical fuse reliably can be found accordingly. After characterizing the program window, the other OTP cells can be programmed with a voltage between the low and high bounds in at least one pulse.

The invention can be implemented in a part or all of an integrated circuit in a Printed Circuit Board (PCB), or in a system (electronic system, computing system, etc.). The programmable resistive memory can be fuse, anti-fuse, or emerging nonvolatile memory. The fuse can be silicided or non-silicided polysilicon fuse, metal fuse, MOS gate, local interconnect, thermally-isolated-active-area fuse, contact fuse, via fuse, or FinFET itself. The anti-fuse can be a gate-oxide breakdown anti-fuse, contact or via anti-fuse with dielectrics in-between. The emerging nonvolatile memory can be Magnetic RAM (MRAM), Phase Change Memory (PCM), Conductive Bridge RAM (CBRAM), Ferroelectric RAM (FeRAM) or Resistive RAM (RRAM). Though the program mechanisms are different, their logic states can be distinguished by different resistance values. The semiconductor can also be fabricated by amorphous, polysilicon, or organic semiconductor thin-film substrate, in the so-called Thin-Film Transistor (TFT) technologies, instead of crystalline silicon technologies, such as bulk or SOI (Silicon-On-Insulator) technologies.

This application incorporates by reference: (1) U.S. Provisional Patent Application No. 62/485,895, filed on Apr. 14, 2017 and entitled “CIRCUIT AND SYSTEM OF ULTRA LOW VOLTAGE AND LOW CURRENT READ FOR PROGRAMMABLE RESISTIVE MEMORIES;” AND (2) U.S. patent application Ser. No. 15/953,422, filed on Apr. 14, 2018 and entitled “LOW POWER READ OPERATION FOR PROGRAMMABLE RESISTIVE MEMORIES.”

The above description and drawings are only to be considered illustrative of exemplary embodiments, which achieve the features and advantages of the present invention. Modifications and substitutions of specific process conditions and structures can be made without departing from the spirit and scope of the present invention.

The many features and advantages of the present invention are apparent from the written description and, thus, it is intended by the appended claims to cover all such features and advantages of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation as illustrated and described. Hence, all suitable modifications and equivalents may be resorted to as falling within the scope of the invention. 

What is claimed is:
 1. An One-Time Programmable (OTP) memory comprises: a plurality of OTP cells, each of the OTP cells including an OTP element coupled to a selector, the selector having a control signal; a plurality of wordlines, each coupled to a plurality of the OTP cells via the control signal of the selector; a plurality of bitlines, each coupled to a plurality of at least one end of the OTP cells; at least one reference OTP cells, each of the reference OTP cells including a reference OTP element coupled to a reference selector, the reference selector having a control signal; at least one reference wordline, each coupled to the reference OTP cells via the control signal of the reference selector; and at least one reference bitline, each coupled to at least one end of the reference OTP cell; wherein the bitlines and reference bitlines are pre-charged and then are discharged at substantially the same time, and wherein the resistance of the OTP element is able to be determined by comparing the discharge rates of the bitline versus the reference bitline.
 2. An OTP memory as recited in claim 1, wherein at least one of the bitline or reference bitline are coupled to at least one dataline or reference dataline through at least one pass gate or reference pass gate for discharge rate comparison, respectively.
 3. An OTP memory as recited in claim 2, wherein at least one of the bitline or dataline is coupled to at least one capacitor or comparator.
 4. An OTP memory as recited in claim 3, wherein at least one of the capacitor is built on MOS gate capacitance.
 5. An OTP memory as recited in claim 3, wherein at least one comparator is built on at least one inverter or Boolean logic gates.
 6. An OTP memory as recited in claim 5, wherein at least one of the comparator changes states when the input voltage is approximately equal to half of the MOS voltage level swing.
 7. An OTP memory as recited in claim 5, wherein at least one of the comparator or reference comparator outputs are coupled to input or enable signals of at least one latch or flip-flop.
 8. An OTP memory as recited in claim 1, wherein at least one of the bitlines are coupled to at least one dataline through at least one pass gate, and wherein each of a plurality of the bitlines or datalines is coupled to a comparator, and wherein the discharging is disabled when at least one of the outputs of comparators changes logic state.
 9. An OTP memory as recited in claim 1, wherein the OTP comprises at least one of the electrical fuse, gate-oxide or dielectric breakdown anti-fuse, or floating-gate OTP.
 10. An OTP memory as recited in claim 9, wherein the electrical fuse comprises at least one of polysilicon, silicided polysilicon, silicide, metal, metal alloy, local intercoonect, thermally isolated active region, FinFET, or MOS gate.
 11. An OTP memory as recited in claim 1, wherein OTP memory is built on a thin-film technology including at least amorphous, polysilicon silicon or organic semiconductor substrate.
 12. An electronic system, comprising: a processor; and a one-time programmable (OTP) memory operatively connected to the processor, the OTP memory includes at least a plurality of OTP cells for providing data storage, each of the OTP cells comprising: a one-time programmable (OTP) element coupled to a selector that has an enable signal; a plurality of wordlines coupled to the enable signals of the selector; a plurality of bitlines coupled to a first end of OTP cells; at least one reference OTP cell comprising a reference OTP resistor coupled to a reference selector that has a reference enable signal; at least one reference wordline coupled to the enable signal; and at least one bitline coupled to at least a first end of the reference OTP cells wherein both the bitline and reference bitline are able to be pre-charged to one voltage supply line and discharged to another supply voltage at substantially the same time, wherein the resistance of the OTP can be determined by comparing the discharge rates of the bitline and the reference bitline.
 13. An electronic system as recited in claim 12, wherein at least one of the bitline or reference bitline are coupled to at least one dataline or reference dataline through at least one pass gate or reference pass gate, respectively.
 14. An electronic system as recited in claim 13, wherein at least one of the bitline or dataline is coupled to at least one capacitor or one comparator.
 15. An electronic system as recited in claim 12, wherein at least one of the comparator is built on at least one inverter or Boolean logic gates.
 16. An electronic system as recited in claim 12, wherein the comparator outputs are coupled to an input and an enable signals, respectively, of a transparent latch or flip-flop to convert the OTP resistance into a logic state.
 17. An electronic system as recited in claim 12, wherein the at least one wordline is disabled and resume pre-charging when the at least one OTP element resistance is determined.
 18. A method for operating a one-time programmable (OTP) memory, the OTP memory including: a plurality of OTP cells, at least one of the OTP cells including a plurality of a OTP element and a selector, the OTP element coupled to a selector; a plurality of wordlines coupled to the enable signal of the selector of at least one OTP cell; a plurality of bitlines coupled to at least a first end of the OTP cells; at least one reference OTP cell, the at least one of the reference OTP cell including at least one reference OTP element coupled to a reference selector; at least one reference wordline coupled to the enable signal of the reference selector of at least one reference OTP cell; at least one reference bitline coupled to at least a first end of the reference resistance cell, wherein the method comprises: charging the bitline and the reference bitline; ceasing charging the bitline and the reference bitline substantially at the same time; subsequently coupling the OTP element and the reference OTP element to the bitline and reference bitline, respectively; monitoring a discharge rate of the bitline and the reference bitline to reach a predetermined voltage threshold; and determining a logic state for the at least one of the OTP cells based on the discharge rates of the bitline to the reference bitline.
 19. A method as recited in claim 18, wherein at least one or more bitlines or reference bitline is coupled to at least one dataline or reference dataline.
 20. A method as recited in claim 19, wherein at least one capacitor or comparator is coupled to at least one cell/reference bitline or dataline.
 21. A method as recited in claim 18, wherein the monitoring the discharge rates of bitline and the reference bitline is via a comparator to convert the discharge voltages into a logic state.
 22. A method as recited in claim 18, wherein the comparator outputs are coupled to an input and an enable, respectively, of a latch or flip-flop.
 23. A method as recited in claim 18, wherein the at least one of the bitline discharging is disabled and resumed charging when at least one of the OTP element resistance is determined.
 24. A method of programming OTP memory as recited in claim 18, wherein the one-time programming of the at least one OTP cell comprises: programming a portion of the OTP memory using initially a relatively low program voltage and gradually incrementally increase the program voltage until the planned OTP cells in the OTP memory are programmed and verified correctly, so as to determine a lower bound of the program voltage; continuously incrementally increasing the program voltage and programming OTP memory until an excessive voltage is identified where at least one OTP cell, either programmed or not, is verified with an error, so as to determine an upper bound of the program voltage; and setting a voltage between the low bound and high bound of the program voltages so as to program the all OTP cells accordingly. 